Conventional coal or gas fired heat exchanger units are well known for generating electricity using turbines. These heat exchanger units often use monolithic ceramic tubes as conduits for the air to be heated in the combustion chamber. As many as three thousand tubes per heat exchanger unit may be used.
Typically, the air running through a turbine is pressurized to 15-20 psi through the heat exchanger tubes and then subsequently highly pressurized to drive the turbine. Although it is less expensive and more efficient to maintain a constant high pressure (e.g., 200 psi) throughout the unit, the inadequacies of monolithic tubes make it impossible to do so.
A common problem associated with monolithic ceramic tubes is catastrophic failure of the tubes due to original flaws or flaws developed during use or by damage from an external source. Typically, a flawed tube subjected to high internal pressure (e.g., 200 psi), blows up or explodes when it fails, creating a shrapnel effect. In many applications, multiple tubes are in close proximity to each other. In the event of failure of one tube, the pieces of the broken tube become projectiles and destroy adjacent tubes. This creates a cascading effect and ultimately results in the destruction of many or all of the tubes in a heat exchanger.
Prior attempts to avoid the catastrophic breakage problems associated with monolithic ceramic tubes include utilization of a ceramic impregnated woven substrate as either an outside wrap or an inner sleeve to constrain pieces of the failed tube sufficiently to prevent damage to the adjacent tubes. The method has several limitations. First, current ceramic fabrics will withstand only about 1,800 degrees Fahrenheit ("F") for any length of time. This is problematic in applications that routinely require temperatures in excess of 1800 degrees F. Second, the methodology requires a flexible ceramic pre-impregnated fabric. The silica component of a ceramic composite may be fluxed by the impurities of coal gas. Consequently, use of the method in a coal fired heat exchanger unit is prohibitive. Third, the inability to match the coefficient of thermal expansion of the currently available commercial exchanger to that of a ceramic system which can be impregnated and subsequently fired causes debonding and cracking of the materials.
Other attempts to avoid the catastrophic breakage problems associated with monolithic ceramic tubes include use of metal tubes rather than ceramic ones. Two primary disadvantages of metal tubes are their temperature limitations and their corrosion limitations. In addition, metal tubes are usually quite heavy and often suffer from fatigue failure. Metals which are more resistant to high temperatures and corrosion are often too expensive to use.